My various projects that may be of interest to others.
Dedicated to my brother Stan, Apr 16, 1955- Dec 4, 2010

Sunday, April 28, 2013

Mini Mill Solid Column Conversion

The original design Sieg Mini Mill( sold as Harbor Freight, Grizzly,
Little Machine Shop, Micro Mark, and others) featured a tilting column
that few people used. The pivot point and the relatively thin column
allowed for a considerable amount of flex in the machine. I found this
especially troublesome when using a boring head, for its single cutter
creates a highly unbalanced load. I could only take very light cuts, or
the column would flex. Fortunately, Little Machine Shop has been
working with Sieg to improve their products sold under LMS' Hi Torque
brand. First they enlarged the table significantly, then they replaced
the motor and noisy gears with a more modern design. That left one area
to improve, the flexible column.

The
new solid column is not just the old column with the pivot removed, but
an entirely new and much heavier casting that weighs 11 pounds more
then the old one. 28 vs. 17. That is a huge 64% increase in weight.
The weight is in additional wall thickness.
At left is the original column, where they apparently tried to make it as thin as possible.

Here
is the new solid column, with wall thicknesses about double the
original, and a substantial mounting flange cast as one piece, rather
than bolted on as before.

Since
there is much less space inside the column, I had to trim the head of
the bolt holding the gas spring in order to get it to fit inside.

The
solid column comes with mounting holes for the Hi Torque electronics,
which mount differently than the Harbor Freight electronics. At left is
the unpainted HF column, showing the mounting holes, and the painted Hi
Torque column below it. I drilled the new column to match the HF, see
below. I also drilled a hole for the gas spring conversion.

Since
I had everything apart, and I am now aware of the added stiffness of
the solid column, I decided to add a spacer to the head which extends
the head out 19 mm and moves it up 20 mm. I made the spacer from a
piece of 3/4 inch aluminum. It has 4 tapped holes and 4 clearance
holes. The spacer bolts to the front half of the head, and then the
rear half bolts to the spacer.

Rear
half of head is then bolted to spacer. This picture shows all that is
left of my original Harbor Freight mini mill. Everything else has been
replaced with parts from Little Machine Shop. Even this remnant of the
HF machine has 4 significant mods:
Belt drive conversion.
Gas spring kit
Spring loaded spindle lock
And now my spacer.
Still to be done is the relocation of the fine feed knob. Eventually this will be replaced by a ball screw Z drive.
What does the inside of a mini mill head look like? Here are some pictures:

The
intermediate gears are no longer used. I left them in place, but locked
their position so that they are disengaged from the main shaft.

The rear half of the head showing the pinion gear.

The finished machine. It is a vastly different mill than the original Harbor Freight mill.

I
put the machine through its paces and while it was much improved, it
still had some head shake under heavy loads. Even though I had the gibs
very tight, when I placed my finger so that I was touching the head and
the column at the same time, I could feel the difference in vibration
between the head and column. This meant the head was moving against the
column. Unlike the gibs for the X and Y axis, where the forces are
mostly pressing the sliding parts together, the head is hanging from the
dovetails on the column. This means that the four 6 mm setscrews are
supporting much of the load, and their contact area is very small. I
added a fifth setscrew, and milled smooth recesses in the back of the
jib where the setscrews press against it. This solved the problem, and
my DRO no longer changes its reading when I tighten the lock..

One
thing I did not expect when making the conversion is that the column is
moved nearly 2 inches forward on the solid column base. Here is a
photo of the two bases showing the location of the column on each. With
the pivot assembly gone, Sieg was able to move the column forward where
the pivot used to be. While this would not be a problem for most
users, the column now interfered with my 4 inch vise, which I had
machined to clear the pivot. I decided that the easiest solution was to
change back to the tilt column base and make an adapter to fit the
solid column to it. I wanted my adapter made from cast iron for the same
reasons machinery is made from it. It has good wear and vibration
dampening properties.

I
scoured my local scrap yard for a suitable piece of cast iron and found
something with nearly perfect dimensions for the job: A 10 pound
barbell weight. At about 8 inches in diameter and nearly an inch thick,
it required a minimum of machining to transform it into an ideal
adapter. First I surfaced both both sides to get them flat and parallel
to each other. Then I cut it square and drilled the holes. I also had
to machine a step in the bottom to match the step on the base.

Prior
to machining the step, I milled the top of the base flat. This part of
the casting was smoothed with body putty, a trick Chinese manufacturers
have been using for years. I milled the surface down until I had clean
metal, then I machined the step in my adapter to match.

I
then bolted on my adapter and drilled the holes for the 8mm column
mounting bolts into the base. I drilled these 14mm deep to accommodate
50mm long bolts. After the holes were drilled, I removed the adapter and
applied JB Weld to the rear surface of the base, and reassembled it.

I did this because I
wanted the adapter and base behave as one piece when I tapped the holes,
and while torquing down the column.

After the JB Weld cured, I tapped the 8mm holes for the column and
assembled the machine. The column is now moved 0.85 inches toward the
rear compared to a stock solid column mill. This is not as far back as
the location of the tilt column due to the fact that the solid column
has a flange in the rear. However, it is a big improvement over the
location of the stock solid column machine.

That created a new problem. The rubber bellows no longer reached the
column, so I made a bracket which clamped to the column dovetail and
shortened the distance the bellows needed to stretch.

A setscrew in the side provides the clamping force in the same manner that jib adjusting screws work.

I
lowered the mounting point to give the Z axis stop a place to rest that
is below the surface of the table. The original bellows mounting screw
holes can be seen above my clamp.

Here
is the Z axis stop in its resting position. Here it does not interfere
with my vise, the end of which is visible on the left.

I
believe the finished machine is at least as good as a stock solid
column mill, and has some advantages. The additional clearance of 0.85
inches may not seem like much, but on a small machine like this, it is
significant. My spacer in the head increases the throat depth by 0.75
inches over the stock machine. The combination of the tilt base and
adapter plate is very rigid, with the 8 pound plate solidly attached
with 7 bolts. This is a viable alternative for those who want the
advantages of the solid column without having to discard their perfectly
good base.

Nice job and thank you for sharing your project. I'm going to start something similar but I had a few questions I was hoping you could clarify. 1) Was the gas spring purchased for the solid column or was this the gas spring modification for the HF model and then adapted for the new solid column? 2) Was the Y axis stock Acme screw long enough for the new base or did this need to be retrofit with an upgrade too?

The gas spring is the same as for the tilting column. I had to cut down the head of the bolt for clearance. The Y axis screw is the same for either the solid column or the large table mill with the tilting column. If you have an HF mill, you will need the longer screw. See my blog post on adding the large table for a side by side comparison between the HF small table and the Little Machine Shop large table. You should be able to do my solid column mod using your HF base. If you want to convert to the large base, you might want to consider buying the whole base and large table assembly from Little Machine Shop.

My husband loves power tools and likes building things. His birthday is coming up and I'm trying to find him the right mill ends that he wants. Thanks for this information, because I don't know much about this kind of stuff. I hope I can find the right mill ends!

Hey Robert great post! I bought the X2 Harbor Freight Mini Mill back in May and am just now getting around to getting it all ready to go. I picked this up brand new a couple months ago and had no idea about the weak column until now. So, I was going down the road of the steel plate mods and whatnot until it was suggested to me to get the Solid Column Conversion kit. The only thing scaring me away is that the holes on the back of the new column are not sorrect for the Control Box from my X2.

So... Long story short I have 2 basic questions then:

1.) Do you know what tap sizes I am going to need for the 4 Control Box Mounting Screws. How about for the 2 Grounding Screws then also?

2.) Lastly, is it ok to use the 4 mounting screws for the control box and the 2 ground screws from the X2 then? Or do I need new of these?